1. Field of the Invention
The present invention relates to a sheet conveyance apparatus for conveying a sheet on which an image is formed.
2. Description of the Related Art
An image forming apparatus for image formation on a sheet has recently been utilized usually in combination with a sheet processing apparatus, which is called a finisher and connected to the body of the image forming apparatus. The finisher includes a sheet alignment unit for aligning sheets discharged from the image forming apparatus body at their side ends into a bundle, and stales or punches or sorts the bundle of sheets. At the sorting process, some finisher functions to offset sheet bundles in a direction perpendicular to a sheet conveyance direction (hereinafter referred to as the width direction) for distinguishing the sheet bundles from one another, and discharge the sheet bundles.
Since conveyed sheets must be aligned prior to being stapled or punched, information is needed that indicates a sheet movement amount required for the sheet alignment. Information on a movement amount of each sheet bundle is also required for the offset operation at the time of discharging the sheet bundles.
To determine the sheet movement amount, the position of a conveyed sheet in the width direction must be determined and the movement amount must be computed before execution of post-processing such as stapling. In Japanese Laid-open Patent Publication No. 2005-156578, it is proposed to detect a sheet end position by a sensor disposed to be movable in the sheet width direction, and compute the sheet movement amount required for the sheet alignment.
In U.S. Pat. No. 7,003,257, it is proposed to detect a sheet end portion by a plurality of sensors disposed on a punching apparatus for being moved together with the punching apparatus during the detection, thereby performing hole-punching with accuracy in a punching process. With this proposal, since the sensors can be moved using drive means of the punching apparatus, the arrangement can be simplified in construction and constructed at low cost.
Recently, it has been demanded that a sheet on which an image is formed by an image forming apparatus be efficiently discharged from a post-processing apparatus. However, with the increase in apparatus size and with the increase in types of sheet processing apparatuses used in combination with the image forming apparatus, a lateral misalignment of a sheet is liable to occur in the post-processing apparatus disposed downstream of the image forming apparatus. To improve the productivity in the arrangement where lateral misalignment of a sheet is liable to occur, the lateral misalignment must be computed in a short time period with accuracy.
In the above described prior art, there are proposed a method for detecting lateral misalignment and a method for accurately detecting a sheet end at the post-processing. However, with the increase in amount of lateral misalignment, the sheet side end is at a location more away from the sensor. Thus, the sensor must be moved for a longer distance to detect the sheet side end, and it takes much time to return the sensor to its original position after the sheet end detection. As a result, if the sheet is conveyed at high speed, the sheet side end cannot be detected in time, and the sensor cannot be returned to its original position before arrival of the next sheet. To obviate this, the sheet conveyance speed must be lowered or the sheet conveyance interval must be increased, posing a problem of reduction in productivity (sheet processing number per unit time).